\begin{tikzpicture}[
    node distance = 2cm and 2cm,
    N/.style = {circle, draw, font=\Large, minimum size=1.1em},
    every edge/.style = {draw, -> , semithick, shorten >=2pt},
    every edge quotes/.style = {auto=right, inner sep=2pt}
    ]
    \node [N] (1) at (0,0) [label=left:{+-}] {1};
    \node [N] (3) at ([xshift=5em]1) {3};
    \path (1) edge [loop above] node [above] {a} (1) %循环
    (1) edge [bend left] node [above] {b} (3)
    (3) edge [bend left] node [below] {a} (1);
\end{tikzpicture}
